This invention generally relates to light sources and, more particularly, is directed to a source of vacuum ultraviolet light, that is, light in the spectral region between 190 nm and 100 nm.
The many different types of vacuum ultraviolet (VUV) light sources heretofore proposed and commercialized are based on more or less the same lighting mechanism. For example, the mercury-xenon lamp widely used in the semiconductor industry for photolithography is based on a discharge phenomenon and therefore has a very broad emission spectrum, mainly from far UV to infrared. Its VUV continuum is weak with the result that the VUV emission efficiency is very low. There are light sources with main emission continua in VUV, for example, deuterium lamps; these are also discharge devices which utilize an arc discharge in deuterium gas at a pressure of several Torr and emit light in the short wavelength range below 400 nm. Deuterium lamps are widely used as a continuous UV spectrum in spectrophotometers, and while exhibiting a high VUV emission efficiency its radiant intensity is too low for industrial applications, such as photolithography, because of the wide spread of its spectrum produced by discharge and low pressure operation.
Another known type of VUV light source utilizes microwave excitation of a rare gas. When argon (Ar), krypton (Kr) or xenon (Xe) is excited with a microwave discharge (2450 MHz) it emits a Hopfield-type continuum peaked according to the gas used as follows: Argon, 106-150 nm; krypton, 126-170 nm; xenon, 150-200 nm. Emission continua also occur at longer wavelengths but they are comparatively weak. Although the structure of the microwave-powered lamp itself is quite simple, the microwave generator for powering the lamp is bulky and expensive and consumes large amounts of power. The radiant intensity achieved by lamps of this type typically is less than 10.sup.16 photons/second with an 800-watt generator. Due to the ionization that occurs in the discharge, the emission spectrum resembles that of rare gas discharge by other excitation methods.
A primary object of the present invention is to provide an improved VUV light source.
Another object of the invention is to provide a light source having high VUV emission efficiency.
Still another object of the invention is to provide a light source having higher radiant intensity at VUV wavelength than most VUV light source currently commercially available.
Yet another object is to provide a VUV light source of simple construction and capable of being operated with simple external circuitry, and which can, therefore, be manufactured at relatively low cost.
Another object of the invention is to provide a VUV light source having sufficiently low power consumption as to not require cooling.